1. Field of the Invention
The present invention relates generally to semiconductor light emitting devices using compound semiconductor materials, and more particularly to a semiconductor light emitting device having a light emitting layer of AlGaInP grown on a GaAs substrate.
2. Description of the Prior Art
AlGaInP materials are direct transition-type materials with the largest bandgap energies among III-V compound semiconductor mixed crystals except nitrides and are receiving much attention as materials for visible light emitting devices in the 550-650 nm band (green to red region). The light emitting devices using the AlGaInP materials with such large direct bandgap energies can produce light emission with much higher brightness than the conventional ones using indirect transition-type materials such as GaP or GaAsP.
FIG. 5 of the accompanying drawings shows, in schematic cross section, one example of conventional AlGaInP light emitting devices. The conventional AlGaInP light emitting device 40 includes an n-type GaAs substrate 11 on which an n-type (Al.sub.x Ga.sub.1-x).sub.0.51 In.sub.0.49 P cladding layer 12 (about 1 micrometer thick), an (Al.sub.y Ga.sub.1-y).sub.0.51 In.sub.0.49 P active layer 13 (about 0.6 micrometers thick), a p-type (Al.sub.z Ga.sub.1-z).sub.0.51 In.sub.0.49 P cladding layer 14 (about 1 micrometer thick) and a p-type current spreading layer 15 (several micrometers thick) are formed successively one above the other. Provided on the top of the p-type current spreading layer 15 a p-side electrode (top electrode) 16, and on the bottom surface of the n-type GaAs substrate 11 an n-side electrode (bottom electrode) 17.
Here, the AlGaInP double heterojunction structure comprising the (Al.sub.y Ga.sub.1-y).sub.0.51 In.sub.0.49 P active layer 13 and the two AlGaInP cladding layers which have the bandgap energies larger than that of the active layer 13, i.e. the n-type (Al.sub.x Ga.sub.1-x).sub.0.51 In.sub.0.49 P cladding layer 12 and the p-type (Al.sub.z Ga.sub.1-z).sub.0.51 In.sub.0.49 P cladding layer 14, constitutes a light emitting layer portion 18, and the (Al.sub.y Ga.sub.1-y).sub.0.51 In.sub.0.49 P active layer 13 functions as a light emitting layer. In the AlGaInP double heterojunction structure, the ratios of Al composition, i.e. x, y and z, in the respective AlGaInP layers meet the following relationships: 0.ltoreq.y.ltoreq.0.7, y&lt;x and y&lt;z.
In a description given below, unless otherwise mentioned specifically, the (Al.sub.x Ga.sub.1-x).sub.0.51 In.sub.0.49 P, (Al.sub.y Ga.sub.1-y).sub.0.51 In.sub.0.49 P and (Al.sub.z Ga.sub.1-z).sub.0.51 In.sub.0.49 P will be referred generically to as "(Al.sub.B Ga.sub.1-B).sub.0.51 In.sub.0.49 P" or simply to as "AlGaInP".
The above-described AlGaInP light emitting device requires a current spreading layer, and more specifically it requires a current spreading layer made of a material different from AlGaInP mixed crystals for a reason described below with reference to FIG. 5. In FIG. 5, current distribution 19 from the p-side electrode 16 is indicated by arrows.
In light emission of the AlGaInP light emitting device, it is desiable that a current from the p-side electrode 16 is spread effectively into the entire region of the AlGaInP active layer 13 to occur light-emission with high efficiency. To achieve this, it is required that the distance (layer thickness) between the p-side electrode 16 and the AlGaInP active layer 13 should be a prescribed value (several micrometers and more).
In the AlGaInP light emitting device, as shown in FIG. 5, the three AlGaInP layers 12 (about 1 micrometer thick), 13 (about 0.6 micrometers thick) and 14 (about 1 micrometer thick) are formed on the GaAs substrate 11 with the generic composition of (Al.sub.B Ga.sub.1-B).sub.0.51 In.sub.0.49 P, wherein the AlGaInP layers are lattice-matched with the GaAs substrate 11. However, it is extremely difficult to form (Al.sub.B.sub.Ga.sub.1-B).sub.0.51 In.sub.0.49 P mixed crystal layers of a total thickness exceeding 4 micrometers without causing deterioration of crystalline qualities.
That is, in order to spread the current effectively from the p-side electrode 16 into the entire region of the AlGaInP active layer 13, the thickness between the p-side electrode 16 and the active layer 13 must be several micrometers and more. However, formation of such a thick layer using AlGaInP materials is almost impossible because of the reason described above.
Therefore, conventionally it has been a practice to form a layer made of materials different from AlGaInP on the p-type AlGaInP cladding layer 14, as a current spreading layer 15, so that current from the p-type electrode 16 can be spread effectively into the entire region of the AlGaInP active layer 13 so as to insure efficient emission of light.
As materials for the p-type current spreading layer 15, an AlGaAs or an AlGaAsP or a GaP which is doped with a p-type dopant consisting of Zn, for example, at a high concentration (such as 3.times.10.sup.18 atoms/cm.sup.3) has been conventionally used.
In general, the double heterojunction structure of the AlGaInP light emitting device 40 is manufactured to have a p-type cladding layer 14 of about 1 micrometer in thickness. However, due to a high diffusion coefficient or diffusivity of Zn, such double heterojunction structure encounters a problem that during the growth of a p-type current spreading layer 15, heavily-doped Zn diffuses from the p-type current spreading layer 15 into the p-type cladding layer 14 and further into an active layer 13. Such diffusion of Zn as a p-type dopant causes (1) difficulty in achieving a stable carrier concentration control in the p-type cladding layer 14, (2) deterioration of the interface between the p-type cladding layer 14 and the active layer 13, and (3) crystallinity-deterioration of the active layer 13, which result in a reduction of the emission intensity of the light emitting device.